My test area is near my home PC , now I have a psu that is a bit overkill for the pc , so I made a wire with molex on one end and a double connector for both carpsu and lcd on the other end , works like a charm.

you could also try a spare battery if you have one , or any ac adapter with 12 volts will do , just make sure it has enough amps output to supply everything your hooking up.

here is a link to calculate the amount of amps you need according to the watts your setup draws.

I'd suggest a typical PC/ATX PSU.
They can be free from discarded PCs else a few dollars from markets etc.
They have a more than copious 5V supply and often a reasonable 12V supply.
They are self contained; reasonably shielded, and have little output noise.
Plenty of (parallel) outputs. Connectors can be used or cut off & wire-joined.

A PC PSU will only put out 12V DC and won't vary like a battery or vehicle supply (eg, 12.6V; 8V - 15V etc) and it will lack a vehicle's electrical noise and spikes/transients.
But it should be be fine for testing basic operation; power consumption; the presence of ground loops; etc.

PS: Compared to a battery and charger - most chargers are very noisy. Some output high spikes (pulses) and others may exceed 15V. A PC PSU is lighter and probably safer and cheaper.

You really need to look at the power supply. I was actually surprised when I was looking at an article that showed how to make a laboratory power supply out of a standard PC power supply that the +12 volt portion of a standard PC power supply supplies far less amperage than the +5 volt portion. Most power supplies will have ratings for each of its power rails and what they can output. You will also never want to run a PC supply without it being loaded down because it can burn up the supply. In the lab supply conversions they install 10watt resistors across certain lines. Apparently a standard PC supply has a load requirement on them to produce a smooth power curve. You also have issues with turning it on and such. Just look up "Converting ATX power supply to benchtop power supply" or something similar and you will find out a bunch of information. The PC power supplies supply 3volts and some negative voltages as well.

Personally I have a Radio Shack power supply. It says it is a +12 supply but actually provides 19volts in a manner similar to an automotive supply. I believe the one I have is a 150 watt supply and I have had no issues running my car pc in the manner it was intended. I do have a on off switch I installed inline to reproduce the ignition switch lead to the computer to test it as if it was in car.

I also wired a cigarette lighter style plug on this so I can add whatever accessories I want to add to try out.